As both a writer and an editor, tools like this are essential. Most writing is done in the context of an organization, like a publication or company. Those organizations have style rules. Rules for things like US or UK spelling, for usage questions where there is no single correct answer. Maybe they start with the Chicago Manual of Style, but add custom rules. Maybe it's the New York Times Manual of Style and Usage. It's just like a style checker for code. It frees writers and editors from having to worry about details to focus on the important and difficult aspects of writing.

What details? Things like "do you use 's for possessives even for a word/name ending in s, or is it s'?" or "oxford comma or no?". Writers argue over these just like programmers argue over tabs v spaces or the place of braces. Sure, if you're just blogging for personal use, go crazy, don't worry about a consistent style, do what you want. When writing for publication, instead of arguing over whether or not it's ok to use certain slang or contractions, the editors just chose a preferred way.

Tools like Vale streamline the process of aligning to a style, just like coding style checkers and autoformatters relieve programmers of the tedium fixing up all the indentation manually.